A power brake system having electronic slip regulation is described, for example, in German Patent Application No. DE 10 2014 205431 A1.
This conventional power brake system has a master brake cylinder that can be actuated by the driver, to which at least one brake circuit having at least one wheel brake is separably connected. Between the master brake cylinder and the wheel brake, there is situated a pressure modulation device having a plunger unit and an electronically controllable pressure adjusting device. The plunger unit has a plunger piston that can be actuated by a drive, which piston is situated in displaceable fashion in a plunger cylinder and which limits a plunger working space in the plunger cylinder whose volume increases or decreases as a function of the direction of the movement of the plunger. The pressure adjusting device adapts the brake pressure provided by the plunger to the slippage conditions that prevail at a wheel of the vehicle associated with a wheel brake. For this purpose, the pressure adjusting device controls, inter alia, a flow of pressure medium from the wheel brake into a return line. This return line is connected to a reservoir of the power brake system for pressure medium. A closed pressure medium circuit results due to the fact that as soon as the plunger piston has reached its outermost point of reversal, or at least has approached this outermost point of reversal, the plunger piston then suctions new pressure medium from the mentioned reservoir by reversing the original direction of motion of the plunger piston. Here, the rest of the brake system is decoupled from the plunger unit through the controlling of corresponding plunger control valves.
Inside the reservoir, atmospheric pressure prevails, and the pressure medium contained there has a separating layer from the ambient atmosphere. For this reason, and in addition because the plunger unit is always reloaded with pressure medium stored upstream in the reservoir, the mentioned pressure medium circuit is referred to as an open pressure medium circuit.
By reversing the direction of movement of the plunger piston when pressure medium is suctioned, theoretically a pressure difference of a maximum of approximately 1 bar relative to atmospheric pressure can be established in the plunger working space. However, the maximum pressure difference is a function of the prevailing ambient conditions, for example the current atmospheric air pressure, weather conditions, and/or elevation above sea level. Accordingly, the pressure difference can turn out to be less, or can be subject to strong fluctuations.
Independent of this, in the suction path of the plunger unit there is provided a check valve in order to block this suction path when the direction of movement of the plunger piston is reversed again after a suction phase, and the plunger unit is operated in a pressure phase or working phase. This check valve is standardly a seat valve in which a valve closing element releases or closes a valve cross-section. The limited valve cross-section of the check valve disadvantageously has a throttling effect on a flow of pressure medium to the plunger unit. In particular at low ambient temperatures, with correspondingly increasing viscosity of the pressure medium, in this way the time is made longer for compensating the pressure medium requirement of the plunger unit, and thus the duration is made longer during which the vehicle brake system can merely hold stable the previously set brake pressure at the wheel brakes, or reduce it. This is because, understandably, during a suction phase of the plunger unit no buildup of brake pressure is possible. The ultimate result is that this lengthens the braking distance of the vehicle.
An object of the present invention is to provide a power brake system such that the duration of a suction phase of the plunger unit is reduced.